Variable speed compressor protection system and method

ABSTRACT

A system and method for a compressor includes a compressor connected to a condenser, a discharge line temperature sensor that outputs a discharge line temperature signal corresponding to a discharge line temperature of refrigerant leaving the compressor, and a control module connected to the discharge line temperature sensor. The control module determines a saturated condenser temperature, calculates a discharge superheat temperature based on the saturated condenser temperature and the discharge line temperature, and monitors a flood back condition of the compressor by comparing the discharge superheat temperature with a predetermined threshold. The control module increases a speed of the compressor when the discharge superheat temperature is less than or equal to the predetermined threshold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/346,220, filed on Nov. 8, 2016 and issuing as U.S. Pat. No.10,077,774, which is a continuation of U.S. patent application Ser. No.13/893,493, filed on May 14, 2013 and issued as U.S. Pat. No. 9,494,158,which is a continuation of U.S. patent application Ser. No. 12/246,959,filed on Oct. 7, 2008 and issued as U.S. Pat. No. 8,459,053. Thisapplication claims the benefit of U.S. Provisional Application No.60/978,258, filed on Oct. 8, 2007. The entire disclosures of each of theabove applications are incorporated herein by reference.

FIELD

The present disclosure relates to compressors, and more particularly, toa protection system for use with a variable speed compressor.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

Compressors may be used in a wide variety of industrial and residentialapplications to circulate refrigerant within a refrigeration, heat pump,HVAC, or chiller system (generically “refrigeration systems”) to providea desired heating or cooling effect. In any of the foregoingapplications, the compressor should provide consistent and efficientoperation to insure that the particular application (i.e.,refrigeration, heat pump, HVAC, or chiller system) functions properly. Avariable speed compressor may be used to vary compressor capacityaccording to refrigeration system load.

Operation of the compressor during a flood back condition isundesirable. A flood back condition occurs when excessive liquidrefrigerant flows into the compressor. Severe flood back can dilute theoil and reduce its lubrication property, leading to potential seizure.Although some mixture of liquid refrigerant and oil in the compressormay be expected, excessive mixture may cause damage to the compressor.

Likewise, operation of the compressor at excessive temperature levelsmay be damaging to the compressor. An overheat condition may damageinternal compressor components including, for example, the electricmotor.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

A system is provided that includes a compressor connected to a condenserand a discharge line temperature sensor that outputs a discharge linetemperature signal corresponding to a discharge line temperature ofrefrigerant leaving the compressor. The system also includes a controlmodule connected to the discharge line temperature sensor. The controlmodule determines a saturated condenser temperature, calculates adischarge superheat temperature based on the saturated condensertemperature and the discharge line temperature, and monitors a floodback condition of the compressor by comparing the discharge superheattemperature with a predetermined threshold. The control module alsoincreases a speed of the compressor when the discharge superheattemperature is less than or equal to the predetermined threshold.

A method is also provided and includes determining, with a controlmodule, a saturated condenser temperature of a condenser connected to acompressor. The method also includes receiving, with the control module,a discharge line temperature signal that corresponds to a discharge linetemperature of refrigerant leaving the compressor. The method alsoincludes calculating, with the control module, a discharge superheattemperature based on the saturated condenser temperature and thedischarge line temperature. The method also includes monitoring, withthe control module, a flood back condition of the compressor bycomparing the discharge superheat temperature with a predeterminedthreshold. The method also includes increasing a speed of the compressorwhen the discharge superheat temperature is less than or equal to thepredetermined threshold.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

FIG. 1 is a schematic illustration of a refrigeration system.

FIG. 2 is a perspective view of a compressor with an inverter drive.

FIG. 3 is another perspective view of a compressor with an inverterdriver.

FIG. 4 is a cross-section view of a compressor.

FIG. 5 is a graph showing discharge super heat correlated with suctionsuper heat and outdoor temperature.

FIG. 6 is a graph showing condenser temperature correlated withcompressor power and compressor speed.

FIG. 7 is a graph showing an operating envelope of a compressor.

FIG. 8 is a graph showing condensing temperature correlated withevaporator temperature and compressor power for a given compressorspeed.

FIG. 9 is a graph showing discharge line temperature correlated withevaporator temperature and condenser temperature.

FIG. 10 is a flow chart showing derived data for a refrigeration system.

FIG. 11 is a schematic of a refrigeration system.

FIG. 12 is a flow chart showing derived data for a refrigeration system.

FIG. 13 is a graph showing mass flow correlated with inverter drive heatloss.

FIG. 14 is a graph showing inverter speed correlated with inverterefficiency.

FIG. 15 is a graph showing a control module with measured inputs andderived outputs.

FIG. 16 is a schematic of a refrigeration system.

FIG. 17 is a cross-section view of a compressor.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses. Itshould be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.

As used herein, the terms module, control module, and controller mayrefer to one or more of the following: An application specificintegrated circuit (ASIC), an electronic circuit, a processor (shared,dedicated, or group) and memory that execute one or more software orfirmware programs, a combinational logic circuit, or other suitablecomponents that provide the described functionality. As used herein,computer readable medium may refer to any medium capable of storing datafor a computer or module, including a processor. Computer-readablemedium includes, but is not limited to, memory, RAM, ROM, PROM, EPROM,EEPROM, flash memory, CD-ROM, floppy disk, magnetic tape, other magneticmedium, optical medium, or any other device or medium capable of storingdata for a computer.

With reference to FIG. 1, an exemplary refrigeration system 5 includes acompressor 10 that compresses refrigerant vapor. While a specificrefrigeration system is shown in FIG. 1, the present teachings areapplicable to any refrigeration system, including heat pump, HVAC, andchiller systems. Refrigerant vapor from compressor 10 is delivered to acondenser 12 where the refrigerant vapor is liquefied at high pressure,thereby rejecting heat to the outside air. The liquid refrigerantexiting condenser 12 is delivered to an evaporator 16 through anexpansion valve 14. Expansion valve 14 may be a mechanical or electronicvalve for controlling super heat of the refrigerant. The refrigerantpasses through expansion valve 14 where a pressure drop causes the highpressure liquid refrigerant to achieve a lower pressure combination ofliquid and vapor. As hot air moves across evaporator 16, the lowpressure liquid turns into gas, thereby removing heat from evaporator16. The low pressure gas is again delivered to compressor 10 where it iscompressed to a high pressure gas, and delivered to condenser 12 tostart the refrigeration cycle again.

With reference to FIGS. 1, 2 and 3, compressor 10 may be driven by aninverter drive 22, also referred to as a variable frequency drive (VFD),housed in an enclosure 20. Enclosure 20 may be near compressor 10.Inverter drive 22 receives electrical power from a power supply 18 anddelivers electrical power to compressor 10. Inverter drive 22 includes acontrol module 25 with a processor and software operable to modulate andcontrol the frequency of electrical power delivered to an electric motorof compressor 10. Control module 25 includes a computer readable mediumfor storing data including the software executed by the processor tomodulate and control the frequency of electrical power delivered to theelectric motor of compressor and the software necessary for controlmodule 25 to execute and perform the protection and control algorithmsof the present teachings. By modulating the frequency of electricalpower delivered to the electric motor of compressor 10, control module25 may thereby modulate and control the speed, and consequently thecapacity, of compressor 10.

Inverter drive 22 includes solid state electronics to modulate thefrequency of electrical power. Generally, inverter drive 22 converts theinputted electrical power from AC to DC, and then converts theelectrical power from DC back to AC at a desired frequency. For example,inverter drive 22 may directly rectify electrical power with a full-waverectifier bridge. Inverter driver 22 may then chop the electrical powerusing insulated gate bipolar transistors (IGBT's) or thyristors toachieve the desired frequency. Other suitable electronic components maybe used to modulate the frequency of electrical power from power supply18.

Electric motor speed of compressor 10 is controlled by the frequency ofelectrical power received from inverter driver 22. For example, whencompressor 10 is driven at sixty hertz electric power, compressor 10 mayoperate at full capacity operation. When compressor 10 is driven atthirty hertz electric power, compressor 10 may operate at half capacityoperation.

Piping from evaporator 16 to compressor 10 may be routed throughenclosure 20 to cool the electronic components of inverter drive 22within enclosure 20. Enclosure 20 may include a cold plate 15. Suctiongas refrigerant may cool the cold plate prior to entering compressor 10and thereby cool the electrical components of inverter drive 22. In thisway, cold plate 15 may function as a heat exchanger between suction gasand inverter drive 22 such that heat from inverter drive 22 istransferred to suction gas prior to the suction gas entering compressor10.

As shown in FIGS. 2 and 3, electric power from inverter drive 22 housedwithin enclosure 20 may be delivered to compressor 10 via a terminal box24 attached to compressor 10.

A compressor floodback or overheat condition is undesirable and maycause damage to compressor 10 or other refrigeration system components.Suction super heat (SSH) and/or discharge super heat (DSH) may becorrelated to a flood back or overheating condition of compressor 10 andmay be monitored to detect and/or predict a flood back or overheatingcondition of compressor 10. DSH is the difference between thetemperature of refrigerant vapor leaving the compressor, referred to asdischarge line temperature (DLT) and the saturated condenser temperature(Tcond). Suction super heat (SSH) is the difference between thetemperature of refrigerant vapor entering the compressor, referred to assuction line temperature (SLT) and saturated evaporator temperature(Tevap).

SSH and DSH may be correlated as shown in FIG. 5. The correlationbetween DSH and SSH may be particularly accurate for scroll typecompressors, with outside ambient temperature being only a secondaryeffect. As shown in FIG. 5, correlations between DSH and SSH are shownfor outdoor temperatures (ODT) of one-hundred fifteen degreesFahrenheit, ninety-five degrees Fahrenheit, seventy-five degreesFahrenheit, and fifty-five degrees Fahrenheit. The correlation shown inFIG. 5 is an example only and specific correlations for specificcompressors may vary by compressor type, model, capacity, etc.

A flood back condition may occur when SSH is approaching zero degrees orwhen DSH is approaching twenty to forty degrees Fahrenheit. For thisreason, DSH may be used to detect the onset of a flood back conditionand its severity. When SSH is at zero degrees, SSH may not indicate theseverity of the flood back condition. As the floodback condition becomesmore severe, SSH remains at around zero degrees. When SSH is at zerodegrees, however, DSH may be between twenty and forty degrees Fahrenheitand may more accurately indicate the severity of a flood back condition.When DSH is in the range of thirty degrees Fahrenheit to eighty degreesFahrenheit, compressor 10 may operate within a normal range. When DSH isbelow thirty degrees Fahrenheit, the onset of a flood back condition mayoccur. When DSH is below ten degrees Fahrenheit, a severe flood backcondition may occur.

With respect to overheating, when DSH is greater than eighty degreesFahrenheit, the onset of an overheating condition may occur. When DSH isgreater than one-hundred degrees Fahrenheit, a severe overheatingcondition may be present.

In FIG. 5, typical SSH temperatures for exemplar refrigerant chargelevels are shown. For example, as the percentage of refrigerant chargein refrigeration system 5 decreases, SSH typically increases.

To determine DSH, DLT may be subtracted from Tcond. DLT may be sensed bya DLT sensor 28 that senses a temperature of refrigerant exitingcompressor 10. As shown in FIG. 1, DLT sensor 28 may be external tocompressor 10 and may be mounted proximate a discharge outlet ofcompressor 10. Alternatively, an internal DLT sensor 30 may be used asshown in FIG. 4. In FIG. 4, a cross-section of compressor 10 is shown.Internal DLT sensor 30 may be embedded in an upper fixed scroll of ascroll compressor and may sense a temperature of discharge refrigerantexiting the intermeshing scrolls.

In the alternative, a combination temperature/pressure sensor may beused. In such case, Tcond may be measured based on the pressure ofrefrigerant exiting compressor 10 as measured by the combination sensor.Moreover, in such case, DSH may be calculated based on DLT, as measuredby the temperature portion of the sensor, and on Tcond, as measured bythe pressure portion of the combination sensor.

Tcond may be derived from other system parameters. Specifically, Tcondmay be derived from compressor current and voltage (i.e., compressorpower), compressor speed, and compressor map data associated withcompressor 10. A method for deriving Tcond based on current, voltage andcompressor map data for a fixed speed compressor is described in thecommonly assigned application for Compressor Diagnostic and ProtectionSystem, U.S. application Ser. No. 11/059,646, Publication No. U.S.2005/0235660. Compressor map data for a fixed speed compressorcorrelating compressor current and voltage to Tcond may be compressorspecific and based on test data for a specific compressor type, modeland capacity.

In the case of a variable speed compressor, Tcond may also be a functionof compressor speed, in addition to compressor power.

A graphical correlation between compressor power in watts and compressorspeed is shown in FIG. 6. As shown, Tcond is a function of compressorpower and compressor speed. In this way, a three-dimensional compressormap with data correlating compressor power, compressor speed, and Tcondmay be derived for a specific compressor based on test data. Compressorcurrent may be used instead of compressor power. Compressor power,however, may be preferred over compressor current to reduce the impactof any line voltage variation. The compressor map may be stored in acomputer readable medium accessible to control module 25.

In this way, control module 25 may calculate Tcond based on compressorpower data and compressor speed data. Control module 25 may calculate,monitor, or detect compressor power data during the calculationsperformed to convert electrical power from power supply 18 to electricalpower at a desired frequency. In this way, compressor power and currentdata may be readily available to control module 25. In addition, controlmodule 25 may calculate, monitor, or detect compressor speed based onthe frequency of electrical power delivered to the electric motor ofcompressor 10. In this way, compressor speed data may also be readilyavailable to control module 25. Based on compressor power and compressorspeed, control module 25 may derive Tcond.

After measuring or calculating Tcond, control module 25 may calculateDSH as the difference between Tcond and DLT, with DLT data beingreceiving from external DLT sensor 28 or internal DLT sensor 30.

Control module 25 may monitor DSH to detect a flood back or overheatcondition, based on the correlation between DSH and flood back andoverheat conditions described above. Upon detection of a flood back oroverheat condition, control module 25 may adjust compressor speed oradjust expansion valve 14 accordingly. Control module 25 may communicatewith or control expansion valve 14. Alternatively, control module 25 maycommunicate with a system controller for refrigeration system 5 and maynotify system controller of the flood back or overheat condition. Systemcontroller may then adjust expansion valve or compressor speedaccordingly.

DSH may be monitored to detect or predict a sudden flood back oroverheat condition. A sudden reduction in DLT or DSH without significantaccompanying change in Tcond may be indicative of a sudden flood back oroverheat condition. For example, if DLT or DSH decreases by apredetermined temperature amount (e.g., fifty degrees Fahrenheit) withina predetermined time period (e.g., fifty seconds), a sudden flood backcondition may exist. Such a condition may be caused by expansion valve14 being stuck open. Likewise, a sudden increase in DLT or DSH withsimilar magnitude and without significant accompanying change in Tcondmay be indicative of a sudden overheat condition due to expansion valve14 being stuck closed. For example, if DLT or DSH increases by apredetermined temperature amount (e.g., fifty degrees Fahrenheit) withina predetermined time period (e.g., fifty seconds), a sudden overheatcondition may exist.

Control module 25 may monitor DSH and DLT to determine whethercompressor 10 is operating within a predetermined operating envelope. Asshown in FIG. 7, a compressor operating envelope may provide maximumflood back and maximum and/or minimum DSH/DLT limits. In addition, amaximum scroll temperature limit (Tscroll) may be provided, in the caseof a scroll compressor. In addition, a maximum motor temperature(Tmotor) may be provided. As shown in FIG. 7, compressor speed andexpansion valve 14 may be adjusted based on DSH and/or DLT to insurecompressor operation within the compressor operating envelope. In thisway, DSH and/or DLT may move back into an acceptable range as indicatedby FIG. 7. Compressor speed adjustment may take priority over expansionvalve adjustment. In some cases, such as a defrost state, it may bedifficult for expansion valve 14 to respond quickly and compressor speedadjustment may be more appropriate.

In the event of a flood back condition, control module 25 may limit acompressor speed range. For example, when DSH is below thirty degreesFahrenheit, compressor operation may be limited to the compressor'scooling capacity rating speed. For example, the cooling capacity ratingspeed may be 4500 RPM. When DSH is between thirty degrees Fahrenheit andsixty degrees Fahrenheit, compressor operating speed range may beexpanded linearly to the full operating speed range. For example,compressor operating speed range may be between 1800 and 7000 RPM.

The function correlating Tcond with compressor speed and power, mayassume a predetermined or constant saturated Tevap. As shown in FIG. 8,the correlation between compressor power and Tcond may be insensitive tovariations of Tevap.

For additional accuracy, Tevap may be derived as a function of Tcond andDLT, as described in commonly assigned U.S. application Ser. No.11/059,646, U.S. Publication No. 2005/0235660. For variable speedcompressors, the correlation may also reflect compressor speed. In thisway, Tevap may be derived as a function of Tcond, DLT and compressorspeed.

As shown in FIG. 9, Tevap is shown correlated with DLT, for variousTcond levels. For this reason, compressor map data for different speedsmay be used.

Tcond and Tevap may be calculated based on a single derivation.

In addition, iterative calculations may be made based on the followingequations:Tcond=f(compressor power,compressor speed,Tevap)  Equation 1:Tevap=f(Tcond,DLT,compressor speed)  Equation 2:

Multiple iterations of these equations may be performed to achieveconvergence. For example, three iterations may provide optimalconvergence. As discussed above, more or less iteration, or noiterations, may be used.

Tevap and Tcond may also be determined by using compressor map data, fordifferent speeds, based on DLT and compressor power, based on thefollowing equations:Tevap=f(compressor power,compressor speed,DLT)  Equation 3:Tcond=f(compressor power,compressor speed,DLT)  Equation 4:

Once Tevap and Tcond are known, additional compressor performanceparameters may be derived. For example, compressor capacity andcompressor efficiency may be derived based on additional compressorperformance map data for a specific compressor model and capacity. Suchadditional compressor map data may be derived from test data. Forexample, compressor mass flow or capacity, may be derived according tothe following equation:Tevap=f(compressor speed,Tcond,mass flow)  Equation 5:

Mass flow may be derived according to the following equation:MassFlow=m0+m1Tevap+m2*Tcond+m3*RPM+m4*Tevap*Tcond+m5*Tevap*RPM+m6*Tcond*RPM+m7*Tevap{circumflexover ( )}2+m8*Tcond{circumflex over ( )}2+m9*RPM{circumflex over( )}2+m10*Tevap*Tcond*RPM+m11*Tevap{circumflex over( )}2*Tcond+m12*Tevap{circumflex over ( )}2*RPM+m13*Tevap{circumflexover ( )}3+m14*Tevap*Tcond{circumflex over ( )}2+m15*Tcond{circumflexover ( )}2*RPM+m16*Tcond{circumflex over ( )}3+m17*Tevap*RPM{circumflexover ( )}2+m18*Tcond*RPM{circumflex over ( )}2+m19*RPM{circumflex over( )}3  Equation 6:

where m0-m19 are compressor model and size specific, as published bycompressor manufacturers.

Compressor map data may be stored within a computer readable mediumwithin control module 25 or accessible to control module 25.

As shown in FIG. 10, a flow chart for derived parameters is shown. Instep 100, Tcond may be derived from compressor power and compressorspeed. In step 101, Tevap may be derived from DLT and Tcond. In step102, capacity/mass flow and a compressor energy efficiency ratio may bederived from Tevap and Tcond. In addition, by monitoring run time instep 103, additional parameters may be derived. Specifically, in step104, load and Kwh/Day may be derived from run time, capacity/mass flow,EER, and compressor power.

By monitoring the above operating parameters, control module 25 mayinsure that compressor 10 is operating within acceptable operatingenvelope limits that are preset by a particular compressor designer ormanufacturer and may detect and predict certain undesirable operatingconditions, such as compressor floodback and overheat conditions.Further, control module 25 may derive other useful data related tocompressor efficiency, power consumption, etc.

Where compressor 10 is driven by a suction cooled inverter drive 22,Tevap may be alternatively calculated. Because Tevap may be calculatedfrom mass flow, Tcond, and compressor speed as discussed above, controlmodule 25 may derive mass flow from a difference in temperature betweensuction gas entering cold plate 15 (Ts) and a temperature of a heat sink(Ti) located on or near inverter drive 22. Control module 25 maycalculate delta T according to the following equation:delta T=Ts−Ti  Equation 7:

Ts and Ti may be measured by two temperature sensors 33 and 34 shown inFIG. 11. Temperature sensor 33 measures the temperature of the heat sink(Ti) and may be incorporated as part of inverter drive 22.Alternatively, temperature sensor 33 may measure a temperature ofsuction gas exiting cold plate 15 and may be located on or near thepiping between cold plate 15 and compressor 10. Temperature sensor 34measures the temperature of suction gas entering cold plate 15.

Control module 25 may determine mass flow based on delta T and bydetermining the applied heat of inverter drive 22. As shown in FIG. 12,mass flow may be derived based on lost heat of inverter drive 22 anddelta T. As shown in FIG. 13, the relationship between mass flow, deltaT and applied inverter heat may be mapped based on test data.

Inverter heat may be derived based on inverter speed (i.e., compressorspeed) and inverter efficiency as shown in FIG. 14.

With reference again to FIG. 12, inputs include compressor speed (RPM)120, compressor current 122, compressor voltage 124, compressor powerfactor 126, Ti 128 and Ts 130. From compressor current 122, compressorvoltage 124, and power factor 126, compressor power 132 is derived. Fromtemperatures Ti 128 and Ts 130, delta T 134 is derived. From RPM 120 andpower, Tcond 136 is derived. Also from RPM 120 and power 132, inverterheat loss 138 is derived. From inverter heat loss, and delta T 134, massflow 140 is derived. From RPM 120, Tcond 136, and mass flow 140, Tevap142 is derived. From Tevap 142 and Ts 130, SSH 144 is derived. From SSH144 and ambient temperature as sensed by ambient temperature sensor 29,DSH 146 is derived. Once DSH 146 is derived, all of the benefits of thealgorithms described above may be gained, including protection ofcompressor 10 from flood back and overheat conditions.

As shown by dotted line 141, Tcond and Tevap may be iterativelycalculated to more accurately derive Tcond and Tevap. For example,optimal convergence may be achieved with three iterations. More or lessiterations may also be used.

As shown in FIG. 15, control module 25 takes as measured inputscompressor speed RPM, inverter drive current, voltage, and power, andheat sink temperatures Ti and Ts. Control module also takes as inputambient temperature as indicated by ambient temperature sensor 29. Asdiscussed above, control module 25 derives from these measured inputsthe outputs of Tcond, Tevap, mass flow, SSH, DSH, and DLT.

As shown in FIG. 16, control module 25 may monitor SLT with SLT sensor35, which may include a combination pressure and temperature sensor maybe used. In such case, Tevap may be measured based on the suctionpressure as measured by the pressure portion of the combination sensor.Further, SSH may be calculated based on SLT, as measured by thetemperature portion of the combination sensor, and Tevap. SLT sensor 34,35 may be located at an inlet to compressor 10 and may sense atemperature or pressure of refrigerant entering compressor 10 subsequentto inverter 22, enclosure 20, or cold plate 15. Alternatively SLT sensormay be located at an inlet to enclosure 20, inverter 22, or cold plate15 and may sense a temperature or pressure of refrigerant entering theenclosure 20, inverter 22, or cold plate 15.

In addition, similar to the calculation of DSH based on DLT describedabove, control module 25 may also calculate SSH. For example, compressorpower, compressor speed, and compressor map data may be used to deriveTcond and Tevap may be derived from Tcond. Once Tevap is derived, SSHmay be derived from SLT and Tevap and used as described above formonitoring various compressor operating parameters and protectingagainst flood back and overheat conditions.

What is claimed is:
 1. A method comprising: receiving, with acontroller, compressor power data; determining, with the controller, asaturated condenser temperature of a condenser connected to a compressorbased on the compressor power data; receiving, with the controller, adischarge line temperature signal that corresponds to a discharge linetemperature of refrigerant leaving the compressor; calculating, with thecontroller, a discharge superheat temperature based on the saturatedcondenser temperature and the discharge line temperature; monitoring,with the controller, a flood back condition of the compressor bycomparing the discharge superheat temperature with a predeterminedthreshold; and increasing a speed of the compressor, with thecontroller, when the discharge superheat temperature is less than orequal to the predetermined threshold.
 2. The method of claim 1 whereinthe predetermined threshold is thirty degrees Fahrenheit.
 3. The methodof claim 1 wherein the controller increases the speed of the compressorwhen the discharge superheat temperature is less than or equal to thepredetermined threshold.
 4. The method of claim 1 further comprisingdecreasing, with the controller, an opening of an expansion valveassociated with the compressor when the discharge superheat temperatureis less than or equal to the predetermined threshold.
 5. The method ofclaim 1 further comprising monitoring, with the controller, a suddenflood back condition by determining whether the discharge superheattemperature decreases by a predetermined amount within a predeterminedtime period.
 6. The method of claim 1 further comprising receiving, withthe controller, a discharge pressure signal corresponding to a dischargepressure of refrigerant leaving the compressor and determining thesaturated condenser temperature based on the discharge pressure.
 7. Themethod of claim 1 further comprising determining, with the controller,the saturated condenser temperature as a function of the compressorpower data and the speed of the compressor.
 8. The method of claim 1further comprising receiving, with the controller, compressor powerdata, determining a saturated evaporator temperature of an evaporatorconnected to the compressor and the condenser as a function of thesaturated condenser temperature, the discharge line temperature, and thespeed of the compressor, and determining the saturated condensertemperature as a function of the compressor power data, the speed of thecompressor, and the saturated evaporator temperature.
 9. The method ofclaim 8 further comprising performing, with the controller, multipleiterations of determining the saturated condenser temperature and thesaturated evaporator temperature to achieve convergence.
 10. The methodof claim 1 further comprising receiving, with the controller, compressorpower data and determining the saturated condenser temperature as afunction of the compressor power data, the speed of the compressor, andthe discharge line temperature.